Phosphor coating composition



United States Patent I r 2,744,012] Patented May 1, 1956 ice 2,744,072PHOSPHOR COATING COMPOSITION No Drawing. Application June 24, 1953,Serial No. 363,935

6 Claims. (Cl. 252301.3)

The present invention relates to compositions for coating vitreoussurfaces with luminescent materials and, more particularly, tocompositions for coating a luminescent material on the inside surface ofvitreous tubes which are to be used as fluorescent lamp envelopes.

In order to use the ultraviolet radiation of a low pressure positivecolumn mercury discharge, the inside surface of fluorescent lampenvelopes are generally provided with a coating of luminescent materialwhich is applied by suspending the material in a liquid vehicle andcoating the inside surface of the envelope with the suspension. Thisfinal suspension which is used as the coating material is known in theindustry as a paint. Since most solvents which are components of thepaint which will not harm the luminescent materials and which may beremoved from the final coating have too low a viscosity to produce asmooth even coating when the suspension consists solely of a luminescentmaterial suspended in the solvent, a viscosity imparting and controllingagent is generally. a component of the paint. The present practice ofthe industry is to use a solution of nitrocellulose in butyl or amylacetate as the viscosity imparting and controlling agent and suspendingmedium for the luminescent material. The nitrocellulose imparts thedesired viscosity 'to the solution so that it is possible to obtain asmooth, even coating of a luminescent material on the vitreous innersurface of the lamp envelope, which nitrocellulose viscosity controllingagent and suspending medium may be burned or baked out after the coatinghas been applied, and the butyl or amyl acetate has been volatilizedfrom the coating. While the present use of a solution of nitrocellulosein butyl or amyl acetate has proved satisfactory, as far as the qualityof coating is concerned, the cost of butyl or amyl acetate iscomparatively high, which is extremely important in a highly competitivefield such as the lamp industry. Any substance which can improve thefinish coating of luminescent materials and still meet the otherrequirements of low cost, facility of removal of the suspending mediumfrom the coating and freedom from any residue of foreign matter whichwill affect the luminescent characteristics of the fluorescent material,is indeed a step forward in the art.

There is disclosed in co-pending application of Robert W. Repsher, Ser.No. 307,390, filed Aug. 30, 1952, entitled Luminescent CoatingComposition & Method of Applying and assigned to the present assignee,the use of a liquid vehicle consisting of a water soluble cellulosebinder to control the viscosity of the paint and thus make possible asmooth, even, low cost coating. I have found that the quality of coatingobtained by the coating composition disclosed by Repsher in theheretofore noted copending application can be readily improved upon atnoadded cost by the addition of an alkalizing agent which simultaneouslyacts as-a dispersing agent.

Therefore the principal object of my invention is to provide aninexpensive paint for applying luminescent material to a vitreoussurface, which paint includes as one component thereof an alkalizing anddispersing agent.

Another object of my invention isto provide an inexpensive water basepaint for applying luminescent materials to vitreoussurfaces, whichpaint consists of a luminescent material suspended in a liquid vehicle,which vehicle has as a component thereof an alkalizing and dispersingagent.

A further object of my invention is to provide an inexpensive paint forapplying luminescent materials to vitreous surfaces, a component ofwhich paint is a dispersing and alkalizing medium that is readilyremovable from the coating Without leaving a residue which will'affectthe luminescent qualities of the luminescent materials.

Yet another object of my invention is to provide a smooth flowing paintconsisting of a luminescent material suspended in a liquid vehicle,which vehicle consists of a solution of a water soluble organic binderand a Water base solvent mixture, a component of which water basesolvent mixture is ammonium hydroxide or an amine.

A further object of my invention'is to provide optimum and allowableranges for the components which comprise my paint.

The aforesaid objects of the invention, and other objects which willbecome apparent as the description proceeds, are achieved by providing asmooth flowing coating composition or paint consisting of a luminescentmaterial suspended in a liquid vehicle, which liquid vehicle consists ofa solution of methyl cellulose and a water base solvent mixture. TheWater base solvent mixture is basically comprised of water and analkalizing'medium consisting of ammonium hydroxide or an amine. A flowagent is also a component of the water base solvent mixture and, ifdesired, a plasticizer may also be added to increase th quality ofcoating. I

Thus basically, my coating composition or paint consists of aluminescent material suspended in a liquid vehicle. The liquid vehicleconsists of a solution of a water soluble organic binder and a waterbase solvent mixture. The water base solvent mixture consists of water,an alkalizer, a flow agent and, if desired, a plasticizer.

As disclosed in the heretofore-noted co-pending Repsher application,methyl cellulose is especially suitable as a binder for use with anaqueous solution of luminescent materials. The aqueous solution ofmethyl cellulose cannot be used alone as a liquid vehicle, however,because the flow and dispersing characteristics of such a solution areunsatisfactory. In order to increase the flow and dispersingcharacteristics of my paint, I have found a flow agent may be added tothe methyl cellulose and water. In addition, and as heretofore noted, Ihave found that the resulting coating of luminescent materials is muchimproved and the luminescent material is much better dispersedthroughout the paint if an alkalizing-dispersing agent is added as acomponent of the paint.

The dispersing or alkalizing agent may be ammonium hydroxide or anyamine whose boiling point (at 760 mm.) is below C. Amines may be broadlydefined as organic compounds whose aqueous solutions are alkaline, and aspecific example of an amine which may be used in my coating compositionis butylamine.

As a flow agent which, as heretofore noted, is also a necessarycomponent of the water base solvent mixture, I may use methyl alcohol ora mono-hydroxy alcohol which is higher in the alcohol series than methylalcohol, such as ethyl and the propyl alcohols, which have a boilingpoint below 100 C. (at 760 mm.) and which are completely water soluble.In the 'heretofore noted co-pending Reps'her application it was notedthat methyl alcohol was unsatisfactory as a fiow'agent. I attribute thisunsatisfactory performance to the free acids normally present incommercially available methyl alcohol. The addition of the alkalizingand dispersing agent, of course, neutralizes the free acids present,thus permitting the use of methyl alcohol as a flow agent, whilesimultaneously 3 better dispersing the luminescent material throughoutthe suspension. In support of this theory, I have found that where ultrapure methyl alcohol is available, it may be used as a flow agent withoutthe addition of the alkalizing or dispersing agent, although it is notcommercially practical to use such ultra pure methyl alcohol.

As heretofore noted, the quality of the coating is still furtherimproved by the addition of a plasticizer to the water base solventmixture. The function of this plasticizer is to flow-out or level thefilm of coating composition, and to increase the yield point of thefilm, thus decreasing the tendency of the film to crack during thedrying or baking treatments, as hereinafter explained.

As a. plasticizer I may use glycerine, triethylene glycol, tetraethyleneglycol, triethylene phosphate, or triethanolamine.

A typical phosphor which may be used in my paint is a 3500 K.halophosph-ate. By explanation of this term, halophosphates arecompounds more or less analogous to the natural mineral apatite, and aresupposed to be represented by a formula such as 3M3(PO4)2-1M'L2, where Lrepresents a halogen or a mixture of halogens and M and M representeither different or identical bivalent metals or mixtures of suchmetals. A 3500 K. halophosphate is a term well-known in the art andrepresents a halophosphate having the same chromaticity as a black bodyat the designated temperature.

In preparing my coating composition, I am governed by variouslimitations. While the ratio of phosphor to binder is in no waycritical, the binder should constitute not less than 1% by weight and nomore than 3% by weight of the luminescent material, if a goodluminiscent coating is to be obtained. If the percent by weight ofmethyl cellulose binder to phosphor is less than 1%, the suspension ofluminescent material in the liquid vehicle will be comparatively poor.If the percent by weight of methyl cellulose to phosphor is more than3%, the methyl cellulose binder will be difiicult to bake out after itis coated. The baking out step in the application of the luminescentcoating to the vitreous surface is hereinafter explained.

The ratio of weight of phosphor to volume of vehicle should be no lessthan 0.25 gram per cubic centimeter, nor greater than 0.5 gram per cubiccentimeter of vehicle. If the ratio is less than 0.25 gram per cubiccentimeter, the phosphor material will be too thinly dispersed, and ifthe ratio is greater than 0.5 gram per cubic centimeter, the phosphormaterial will not be sutficiently dispersed to give a good coating.

I have found that the viscosity of my paint should fall within the rangeof 100 to 200 cps. If the paint viscosity is greater than 200 cps., theresulting luminescent coating will be too thick, and if the paintviscosity is below 100 cps., the resulting luminescent coating will betoo thin.

The viscosity of methyl cellulose which is commercially available variesfrom 4000 to cps. I have found that 4000, 1500 and 400 cps. methylcellulose is suitable for use as a binder in my coating composition. Ifa commercially available methyl cellulose having a viscosity below 400cps. is used, the resulting percentage by weight of methyl cellulose tophosphor will exceed the 3 limitation if a paint viscosity of 100 cps.is to be obtained, and the ratio of weight of phosphor to volume ofluminescent material is not to exceed 0.5 gram per cc. vehicle.

A sufficient amount of flow agent and Water is included in the finalpaint to give it a viscosity which falls within the heretofore recitedlimitations. There is no sure way of predicting in advance just how muchflow agent and water will be required for each paint formulation, but ifthe foregoing limitations of percent by weight of binder to phosphor,phosphor to vehicle and paint viscosities are used as a guide, theamount of flow agent and water in each paint formulation may easily beemperically determined by any man skilled in the art. The percentage byvolume of flow agent to volume of liquid vehicle should be at least 30in order that the proper flow characteristics be maintained. The upperlimit of percentage by volume of flow agent to volume of paint will varygreatly, depending upon the concentration of methyl cellulose used andthe final paint viscosity which is to be obtained. It is obvious thatthe less flow agent I use, the more Water I can use to reach the desiredfinal paint viscosity. By using a greater percentage of water, my paintbecomes cheaper which, as heretofore pointed out, is a most importantconsideration.

The percentage of the ammonium hydroxide or amine alkalizing-dispersingagent should be at least 3% by volume of the total liquid vehicle, inorder to insure an alkaline solution and to insure proper dispersing ofthe phosphor throughout the paint. The percentage by volume of theliquid vehicle of alkalizing-dispersing agent should not exceed 10% inorder to maintain flow characteristics, and in no case should the pH ofmy final paint exceed 12, in order to maintain the solution.

Where I have designated ammonium hydroxide, I refer to a 28% solution.This concentration may be varied, if desired.

Regarding the amounts of the heretofore-noted plasticizers which I mayuse, if desired, I am governed by the dictates of cost vs. improvementin coating composition. Based on these considerations I have found iteconomically impractical to use plasticizers in amounts which exceed 25%by volume of the liquid vehicle. In addition, if the ratio by volume ofplasticizer to liquid vehicle is greater than 1 to 4, the paint will dryvery slowly and the manufacturing difiiculties in appying theluminescent coating on the vitreous surface will be greatly increased.

Thus the foregoing limitations which control the amounts of theindividual components in my final paint may he briefly summarized asfollows:

Percent by weight of methyl cellulose to The following specific examplesof coating compositions are additional illustrations of the presentinvention and are not to be considered as limiting the scope thereof:

EXAMPLE I Percent Relation- Amounts ships Phosphor 3 grams..- 0.29g./cc. vehicle. Methyl cellulose 01,000 cps.) 3 2% 0t phosphor.

so 11 ion. Ammgnium hydroxide (28%, solu- .5 cc 4.8% 0! vehicle.

1011 Isopropyl alcohol 7 cc 67% of vehicle.

EXAMPLE II Phosphor 3 grams.-. 0.27 g./cc. vehicle. Methyl cellulose(4,000 cpsJ. 3 2%0! phosphor.

so u ion. Apjnmgnium hydroxide (28% solu- 1 cc 9%ot vehicle.

ion Isopropyl alcohol 7 cc 64% of vehicle.

EXAMPLE III Percent Relation- Amounts ships Phosphor 4.6 grams. 0.42g./cc. vehicle. Methyl cellulose (4,000 cps.) 3 1.3% of phosphor.

so u D. Antimonium hydroxide (28% solu- 1 cc 9% of vehicle.

ion

Methyl alcohol 7 cc 64% of vehicle.

EXAMPLE IV Phosphor 3 grams... 0.27 g./cc. vehicle. Methyl cellulose(4,000 cps.) 5 001.11% 1.7% of phosphor.

so u ion. Ainmonium hydroxide (28% solulcc 9% of vehicle.

ion

Isopropyl alcohol 4 cc 36% of vehicle. 'Iriethylene glycol (orequivalent 1 cc 9% of vehicle.

- plasticizer).

EXAMPLE V Phosphor 3 grams... 0.27 gJcc. vehicle. Methyl cellulose(4,000 cps.) 3 cc. 2% 2% of phosphor.

Butylamine 9% of vehicle.

Isopropyl alcohol 64% of vehicle.

In preparing the coating composition, I first admix by ball-milling forabout one hour, the luminescent material and aqueous solution of methylcellulose in amounts within the ranges as hereinbefore specified. Theflow agent, alkalizer-dispersing agent and plasticizer, if desired, arethen added in the heretofore-noted ranges. A desir able viscosity hasbeen found to be about 150 cps. As

heretofore noted, the viscosity of the paint may be gov-' erned bycontrolling the amount of flow agent and water which is included in thepaint. Knowing the limitations in amounts of components of the paint,other than the flow agent, the required amount of flow agent and waterto result in the desired viscosity may be readily empirically determinedby any man skilled in the art. The resulting mixture of phosphor,binder, water, flow agent, and alkalizing-dispcrsing agent is thenball-milled for 8 hours to form a suspension of luminescent material inthe liquid vehicle, which vehicle as heretofore noted, consists ofwater, a flow agent, binder and alkalizing-dispersing agent. If desired,the plasticizer may be added at the same time as the flow agent, inamounts as heretofore noted.

As an alternative method of preparing the coating com position, thepreliminary ball-milling of phosphor and aqueous solution of methylcellulose may be omitted and all components of the final coatingcomposition may be milled together for about eight hours to form thepaint.

In applying my paint to the inside of the fluorescent lamp envelope, anyconventional means of application may be used, such as flowing orflushing. The coating is then dried and the methyl cellulosesubsequently removed by baking in the presence of oxygen.

In drying the coating, it is preferable in order to eliminate streaksand poor coating textures, to carefully control the conditions of airvelocity, temperature and humidity while drying the coating prior toburning out the methyl cellulose binder. Air velocities of 300 to 350'per minute have been found satisfactory, and the conditions of air flow,temperature and humidity should be so controlled that the envelopes dryin less than one hour.

After the paint is dried, the methyl cellulose is baked out of theremaining coating by heating to a temperature between approximately 950F. to 1250 F., until the methyl cellulose is completely baked out. Thefluorescent lamp is then completed by sealing the mounts into the endsof the phosphor-coated tube, exhausting, and filling with the propergas, such as argon and mercury vapor, as is'common in the art.

It will be recognizedthat the objects of the invention have beenachieved by providing a coating composition and method of preparationwhich is economical to use 6 v and which produces an excellent coatingfor use in applying a luminescent coating to a vitreous surface.

While in accordance with the Patent Statutes one best known embodimentof the invention has been illustrated and described, it is to beparticularly understood that the invention is not limited thereto orthereby.

I claim:

1. A coating composition for applying luminescent materials to vitreoussurfaces comprising a phosphor suspended in a liquid vehicle, saidliquid vehicle comprising methyl cellulose, water, a completely watersoluble mono-hydroxy alcohol having a boiling point below that of Waterand an alkalizing-dispersing agent selected from the group consisting ofammonium hydroxide and a water soluble amine whose boiling point isbelow that of water, the pH of said composition not exceeding 12, thepercentage by weight of said methyl cellulose to said phosphor beingfrom 1 to 3, said phosphor being suspended in said liquid vehicle in theproportions of from 0.25 to 0.5 gram per cubic centimeter, thepercentage by volume of said alkalizing-dispersing agent to said vehiclebeing from 3 to 10, the percentage by volume of said flow agent to saidvehicle being not less than 30, and the proportions of said flow agentand water in said composition being such that the viscosity of saidcomposition at 20 C. falls within the range of to 200 cps.

2. A coating composition for applying luminescent materials to vitreoussurfaces comprising a phosphor suspended in a liquid vehicle, saidliquid vehicle comprising methyl cellulose, water, a completely watersoluble' mono-hydroxy alcohol having a boiling point below from thegroup consisting of ammonium hydroxide and a water soluble amine whoseboiling point is below that v of water and a plasticizer, the pH of saidcomposition not exceeding 12, the percentage by Weight of said methylcellulose to said phosphor being from 1 to 3, said phosphor beingsuspended in said-liquid vehicle in the proportions of from 0.25 to 0.5gram per cubic centimeter, the percentage by volume of saidalkalizing-dispersing agent to said vehicle being from 3 to 10, thepercentage by volume of said flow agent to said vehicle being not lessthan 30, the percentage by volume of said plasticizer to said liquidvehicle being from 0 to 25, and the proportions of said flow agent andwater in said composition being such that the viscosity of saidcomposition at 20 C. falls within the range of 100 to 200 cps.

3. A coating composition for applying luminescent materials to vitreoussurfaces comprising a phosphor suspended in a liquid vehicle, saidliquid vehicle comprising methyl cellulose, water, isopropyl alcoholand-28% ammonium hydroxide, the percentage by weight of said methylcellulose to said phosphor being about 2, said phosphor being suspendedin said liquid vehicle in the proportions of about 0.29 gram per cubiccentimeter, the percentage by volume of said ammonium hydroxide to saidvehicle being about 4.8, and the percentage by volume of said isopropylalcohol to said vehicle being about 67.

4. A coating composition for applying luminescent materials to vitreoussurfaces comprising a phosphor suspended in a liquid vehicle, saidliquid vehicle comprising methyl cellulose, water, isopropyl alcohol and28% ammonium hydroxide, the percentage by weight of said methylcellulose to said phosphor being about 2, said phosphor being suspendedin said liquid vehicle in the proportions of about 0.27 gram per cubiccentimeter, the percentage by volume of said ammonium hydroxide to saidvehicle being about 9, and the percentage by volume of said isopropylalcohol to said vehicle being about 64.

5. A coating composition for applying luminescent materials to vitreoussurfaces comprising a phosphor suspended in a liquid vehicle, saidliquid vehicle comprising methyl cellulose, water, methyl alcohol and28% ammonium hydroxide, the percentage by weight of said methylcellulose to said phosphor being about 1.3, said phosphor beingsuspended in said liquid vehicle in the proportions of about 0.42 gramper cubic centimeter, the percentage by volume of said ammoniumhydroxide to said vehicle being about 9, and the percentage by volume ofsaid methyl alcohol to said vehicle being about 64.

6. A coating composition for applying luminescent materials to vitreoussurfaces comprising a phosphor suspended in a liquid vehicle, saidliquid vehicle comprising methyl cellulose, water, isopropyl alcohol,triethylene glycol and 28% ammonium hydroxide, the percentage by weightof said methyl cellulose to said phosphor being about 1.7, said phosphorbeing suspended in said liquid vehicle in the proportions of about 0.27gram per cubic centimeter, the percentage by volume of said ammoniumhydroxide to said vehicle being about 9, the percentage by volume ofsaid isopropyl alcohol to said vehicle being about 36, and thepercentage by volume of said triethylene glycol to said liquid vehiclebeing about 9.

References Cited in the file of this patent UNITED STATES PATENTS2,362,761 Med] Nov. 14, 1944 2,429,404 Dixon Oct. 21, 1947 2,514,131Kaszuba July 4, 1950 OTHER REFERENCES Methocel-Dow Chem. Co.,Publication, 60 page pamphlet-4949, pp. 4, 13-18. (Copy in ScientificLibrary.)

1. A COATING COMPOSITION FOR APPLYING LUMINESCENT MATERIALS TO VITREOUSSURFACES COMPRISING A PHOSPHOR SUSPENDED IN A LIQUID VEHICLE, SAIDLIQUID VEHICLE COMPRISING METHYL CELLULOSE, WATER, A COMPLETELY WATERSOLUBLE MONO-HYDROXY ALCOHOL HAVING A BOILING POINT BELOW THAT OF WATERAND AN ALKALIZING-DISPERSING AGENT SELECTED FROM THE GROUP CONSISTING OFAMMONIUM HYDROXIDE AND A WATER SOLUBLE AMINE WHOSE BOILING POINT ISBELOW THAT OF WATER, THE PH OF SAID COMPOSITION NOT EXCEEDING 12, THEPERCENTAGE BY WEIGHT OF SAID METHYL CELLULOSE TO SAID PHOSPHOR BEINGFROM 1 TO 3, SAID PHOSPHOR BEING SUSPENDED IN SAID LIQUID VEHICLE IN THEPROPORTIONS OF FROM 0.25 TO 0.5 GRAM PER CUBIC CENTIMETER, THEPERCENTAGE BY VOLUME OF SAID ALKALIZING-DISPERSING AGENT TO SAID VEHICLEBEING FROM 3 TO 10, THE PERCENTAGE BY VOLUME OF SAID FLOW AGENT TO SAIDVEHICLE BEING NOT LESS THAN 30, AND THE PROPORTIONS OF SAID FLOW AGENTAND WATER IN SAID COMPOSITION BEING SUCH THAT THE VISCOSITY OF SAIDCOMPOSITION AT 20* C. FALLS WITHIN THE RANGE OF 100 TO 200 CPS.